In recent years car, car design has increasingly focused on safety aspects including the performance of the car in crash situations.
In order to improve the safety of driver and passengers, modern cars comprise an increasing amount of safety devices. Many of these safety devices are aimed at improving safety during crashes. One such safety device is an air bag which is activated during a crash to protect the driver and passengers. Currently, cars typically comprise between one and eight airbags and it is likely that this number will increase in the future.
It is of the outmost importance that safety devices such as air bags are reliably activated in the event of a crash. Furthermore, it is important that the air bags are only activated during a crash, as an unintended activation of an air bag may disturb a driver and possibly cause an accident.
An air bag is typically activated by an activation element known as a squib. Different types of squibs exists but typically they are all activated by a short pulse of significant energy. For example, one type of squib comprises a very fast heating element which when applied the high energy pulse almost instantly generates a very high temperature. This ignites a small charge which sets of sodium azide resulting in the generation of a large volume of nitrogen gas filling the air bag.
In order to ensure a reliable air bag operation, it is critical that a suitable drive circuit is used for generating the activation pulse. FIG. 1 illustrates a simplified air bag activation circuit in accordance with prior art.
FIG. 1 illustrates a squib 101 coupled to a drive circuit 103. The drive circuit 103 is implemented in a single Application Specific Integrated Circuit (ASIC) and comprises functionality for generating the activation pulse which activates the squib 101. More specifically, the drive circuit 103 comprises a high side switch FET (Field Effect Transistor) 105 and a low side switch FET 107. During normal operation, where the air bag is passive, the high side FET 105 and the low side FET 107 are both in an off state and no current can flow through the squib. The use of two switch transistors in series provides increased reliability and failure prevention. Particularly, if either one of the switch FETs short circuits, this will not result in an activation of the air bag as the other switch FET will be in the off state.
The high side FET 105 is controlled by a high side control circuit 109 and the low side FET 107 is controlled by a low side control circuit 111. The low side control circuit 111 produces a signal which switches the low side FET 107 off during normal operation and on if the air bag is being activated. The high side control circuit 109 also controls the high side FET 105 to be off during normal operation and on during air bag activation. However, rather than simply switching the high side FET 105 fully on, the high side control circuit 109 also controls the signal to limit the current to the squib.
Typically, the current through the high side FET 105 is limited to around 2 A. Typically, the same energy supply is used for a plurality of air bags and the current limitation prevents that this energy supply is used up by a short circuit in one air bag. For example, during a crash, the upper squib end may be short circuited to ground. If the current through the high side FET 105 is not limited, the resulting current would become exceedingly high thereby quickly draining the energy supply and possibly preventing the activation of other air bags.
Typically, the drive circuit 103 is not directly connected to the energy supply. Rather, a power switch transistor known as a safing FET 113 is coupled in series with the drive circuit 103. The safing switch 113 is generally an external discrete component. The safing FET 113 provides further failure prevention by providing additional redundancy in the air bag activation operation.
Specifically the operation of the safing FET 113 is controlled by a control circuit 115 in response to different detector inputs than used for activating the drive circuit. Typically the safing FET 113 is controlled by a completely different processing unit based in a different crash detection algorithm and sensor input than for the drive circuit. Thus, the air bag is only activated if both redundant evaluations detect the occurrence of a crash in which case the high side FET 105 and the low side FET 107 of the drive circuit as well as the safing FET 113 are switched on. The safing FET 113 is operated as a simple on/off switch. In some applications several safing FETs are used to provide independent safety switches for different drive circuits. For example, each air bag may be provided with its own safing FET.
The safing FET 113 is coupled to a reverse flow blocking diode 117. It is an inherent feature of the manufacturing of FETs that a reverse parasitic diode 119, 121, 123 is connected between the source and drain.
The reverse flow blocking diode 117 is connected to a capacitor 125 which provides the energy supply to the activation circuit. The capacitor 125 is mounted in close proximity to the air bag activation circuit and ensures that energy may be provided to the air bag activation circuit even if the connection to the battery is broken during the crash. However as the capacitor 125 may be discharged, for example after the car has been switched of for a given duration, an electrical path exists from the upper end of the squib to ground through the capacitor 125 and the parasitic diodes 119, 121.
Accordingly, in the absence of the blocking diode 117, a short circuit resulting in a voltage being applied to the lower end of the squib would result in a current flowing through the squib and thereby activating the air bag. The blocking diode effectively breaks this path. The blocking diode may typically be common to a plurality of drive circuits.
A number of disadvantages are associated with the prior art arrangement of FIG. 1.
Firstly, the requirement for an external safing FET tends to increase the cost and complexity of the arrangement. Furthermore, the safing FET tends to be relatively bulky and as the FET is external to the drive circuit, it requires additional operations during manufacturing.
Furthermore, the prior art design results in a significant energy dissipation in the high side FET 105 which accordingly must be relatively large. Specifically, the energy stored in the reservoir capacitor is given by
  E  =            1      2        ⁢          C      ·              V        2            where C is the capacity of the capacitor and V is the voltage over the capacitor. Hence, in order to store sufficient energy to ensure that the squib is activated, while maintaining the size and cost of the capacitor acceptably low, it is required that the capacitor is charged to a relatively high voltage. Typically, the capacitor is charged to a voltage of around 35-36V.
During activation, the low side FET 107 is fully switched on resulting in a typical voltage drop of less than 2V. Furthermore, the impedance of the switch is relatively low resulting in a typical voltage drop of less than 2V. The voltage drop over the blocking diode 117 is typically around 1V. Furthermore, the safing FET 113 is fully switched on during activation resulting in a typical voltage drop of around 1 V (the on resistance of the safing FET 113 is typically lower than that of the low side FET 107). Accordingly, during the current limiting operation of the high side FET 105, the voltage drop from drain to source is typically in the order of 30V. Typically the current is limited to around 2 A and the squib is fired in typically 2 ms. Therefore, the energy dissipation in the high side FET 105 during activation is around 30V·2 A·2 ms=120 mJ. This energy needs to be absorbed by the high side FET 105 without resulting in a thermal shutdown of the FET. In order to meet this energy requirement, it is necessary that the high side FET 105 is physically large.
However the requirement for a large FET has significant impact on the ASIC cost. Furthermore, as the required size depends on the energy absorption requirement, the design cannot take full advantage of the advances in ASIC manufacturing technology. For example, as improvements in lithography processing are achieved, smaller transistors can be formed resulting in smaller areas being required for circuits, This allows a higher integration and may allow more circuitry to be included in the same ASIC.
Another disadvantage of the prior art arrangement of FIG. 1 is that the blocking diode 117 introduces a significant voltage drop. This voltage drop results in an energy loss which must be compensated by an increase in the capacity of the capacitor. Furthermore, the blocking diode 117 is typically common for a plurality of air bags and thus carries a vary large current during air bag activation. For example the blocking diode 117 may be common for eight air bags thus conducting a typical current of up to 16 A during a crash. Accordingly, the blocking diode 117 is a relatively large discrete component requiring additional operations during assembly of the arrangement.
Hence, an improved system for activating a car safety device would be advantageous and in particular a system allowing for increased flexibility, increased performance, increased integration, improved reliability, reduced cost, reduced size and/or improved energy absorption or dissipation would be advantageous.